The increasing demands for efficiency in gas turbine engines have resulted in a demand for materials capable of withstanding more severe operating conditions. In particular, increased strength is required for certain applications along with the resistance to hot corrosion.
U.S. Pat. No. 3,494,709 describes the fabrication of gas turbine components in single crystal form for improved performance. U.S. Pat. No. 3,567,526 describes how certain mechanical properties can be improved by the essential elimination of carbon from single crystal superalloy articles U.S. Pat. No. 4,116,723 describes a heat treatment applicable to single crystal superalloy articles for enhancing their properties. U.S. Pat. No. 3,619,182 describes a moderate strength superalloy, commercially known as In 792, having superior corrosion reistance. Copending application U.S. Ser. No. 336,002 filed on Dec. 30, 1981 discloses surprising and unexpected mechanical property improvements resulting from the fabrication of carbon free IN 792 type composition in single crystal form. The contents of U.S. Pat. No. 3,619,182 and copending commonly owned U.S. application Ser. No. 336,002 are incorporated herein by reference.
The present invention arose from an investigation of the effects of the minor elements carbon, boron, zirconium and hafnium on the properties of certain commercial alloys in single crystal form (the major function of these minor elements appears to involve grain boundary strengthening). As described in copending application U.S. Ser. No. 336,002, it had previously been determined that fabrication of an alloy known as IN 792 (described in U.S. Pat. No. 3,619,182) in single crystal (but without grain boundary strengtheners) form provided substantial and unexpected benefits in mechanical properties. The single crystal IN 792 articles evaluated had no intentional additions of carbon, boron, zirconium or hafnium. In the course of the investigation of the effects of the minor elements on IN 792, it was observed that adding small amounts of carbon, i.e. 0.10 wt. % to IN 792 single crystals substantially improved the hot corrosion resistance but at the same time substantially reduced the mechanical properties of the material. The improvement of the hot corrosion resistance was completely unexpected and is not understood. As a further step in the investigation, additions of tantalum were made to the basic IN 792 composition in coordination with the added carbon and it was found that when the added tantalum and carbon contents were balanced (to tie up the carbon as tantalum carbide) a good combination of improved mechanical properties and improved corrosion resistance resulted.
Unless otherwise indicated, all percent figures in this application are weight percent values.